This invention is concerned with the electrical switch art and more particularly with components in snap action mechanisms, the forming thereof and the mechanisms formed thereby.
More specifically this invention relates to a precision compact elegant combination switch blade and contact means which is exceedingly useful in snap switch mechanisms--particularly very tiny switch mechanisms which are denominated as being subminiature or sub-subminiature and the switch formed using the combination switch blade and contact means. (It is to be noted that the terms subminiature and sub-subminiature are inexact and cover a wide and overlapping range of sizes. The term "subminiature" will be used hereafter to denominate those switches which are among the smallest mechanically actuatable switches.)
Subminiature switches have been made heretofore. However these prior art subminiature switches do not fall in the category of being "high performance" switches. They are deficient in not being able to meet one or more of the following characteristics --relatively large current capacity (1 Amp. resistive at 28 vdc or 115 vac, 60 Hz); high precision (movement differentials, pretravel, overtravel, all measured in terms of thousandths of one inch); ruggedness (able to withstand major shock forces, temperature extremes, tens of thousands of actuations, etc.) and reliability even when exposed to momentary short circuits of 50 or more times rated current carrying capacity.
It is the nature of switching of electrical currents that there is a migration of the physical material of which the contact areas are comprised upon making and breaking of contact. Heat is also generated upon making and breaking due to arcing. The amount of degradation of the contact areas by the heat and migration of the contact material is, among other things, a function of the cross-sectional areas involved and in the amount of current passing or being switched. The higher the current density the greater the degradation upon switching. The smaller the switch the more difficult it is to prevent severe contact surface degradation or even welding of the contact surfaces.
In small switches of the subminiature size it is very difficult to have a relatively high current carrying capacity and provide sufficient mass in the movable contact to accept the inherent erosion of the contact surface while providing heat conduction away from the contact area. Subjecting prior art switches to short circuit currents of 50 amps or more for a millisecond or more, as called for in standard military specification tests, generally causes severe degradation or destruction of the switches. Also repeated switching of one amp. of current at useful ranges of 28 vdc or 115 vac, 60 Hz also causes denigration of the contact surfaces to the point of unacceptable contact resistance ranges.
An important factor in high precision, tiny, rugged switches is the construction of the movable contact member. Another significant factor is the contact force between the contacting surfaces after switching. The force is applied to the movable contact to cause it to remain in switched position so as to be able to withstand vibration and extreme shock. The means by which force is applied to the movable contact as well as the construction of the movable contact per se are important areas of precision high performance switches.
In sizes larger than subminiature, two general types of snap action switches of the high precision high performance momentary single break type are presently extant in the prior art. One type is the flexible flat blade type having compression and tension arms with an actuator engaging the tension arms to cause an overcentering action of the movable contact mounted on the end of the blade. The prior art blade is less than 0.002 inches thick in the very small switches, and does not have enough cross section area for carrying current, particularly for meeting short circuit specification testing.
The other type of prior art blade is used with a coil spring and is a generally rigid pivotal contact carrying member, the cooperation being such that an actuator engages the coil spring to cause the line of action of the spring to overcenter and thereby move the movable contact mounted on the rigid contact carrying member. The coil spring type of switch has an advantage of being able to apply substantially more force to the movable contact in both its at rest position and when it has been moved to its overcentered position. This contact force is important to give the characteristic of ability to withstand extreme conditions of shock and vibration. The contact force also is important in preventing contact degradation. However and importantly, mere downsizing of these larger than subminiature size switches does not provide an acceptable high performance product of subminiature size.
Pivotal action coil springs for moving a momentary switch pivotal movable contact member, and of the single break type where the actuator actually engages the coil spring, have been known for almost 80 years. In the 1950's more modern versions of electrical switches were developed as exemplified by F. N. Anderson et all, Pat. #3,073,923 which issued Jan. 15, 1963. The switches of this type were variously refined over time and the most recent and sophisticated is of the type shown in Lewandowski et al, Pat. #4,673,778 issued June 16, 1987 and assigned to the Cherry Corporation of Waukegan, Ill. This latest Cherry switch, while well designed and sophisticated, cannot be properly called a high performance switch in that it cannot withstand rugged electrical or mechanical environments and cannot handle high currents with precision. Nor will the beefing up of the switch provide a rugged high performance switch.
In December of 1969 a military specification MIL S 8805/94(AR) was published for a very tiny general purpose high precision subminiature switch, which specification incorporates and refers to Mil S 8805 as updated from time to time, and which in turn incorporates several other specifications and standards. This specification 8805/94 sets forth requirements for a very tiny high performance snap action switch. For the 19 years since its publication, the 8805/94 specification has never been successfully approached and further, the industry has not provided a switch of slightly modified specifications but meeting the essential criteria contained in 8805/94.
The 8805/94 switch requirements called for a tiny in physical size switch which, excluding the terminals and the movable operating button, have an outer envelope dimensions of 0.100 inches plus or minus 0.007 inches in width; 0.300 plus or minus 0.010 inches length; and a height of 0.250 inches plus or minus 0.025 inches when including the raised cover area surrounding the actuator button. The electrical specification requires the ability to switch at least 1 Ampere resistive (0.5 Amperes inductive) at 28 vdc and 115 vac, 60 Hz at sea level with an electrical endurance of 25,000 cycles. In addition there is a requirement of a mechanical endurance of 100,000 cycles with a contact resistance after 25,000 cycles not to exceed 25 milliohms. After 100,000 cycles the contact resistance must not exceed 40 milliohms. The specification requires that the switch withstand defined very short durations of short circuits of 100 Amps. at 6 vdc and remain operative as defined.
The somewhat conflicting concepts of tiny but rugged is exacerbated by the additional requirements that the switch also must be able to withstand 100 G forces of shock, be able to meet vibration tests through a range of 10 through 2,000 Hz and be operable over a temperature range of 180 degrees Centigrade (-55C to +125C). The distances between contact surfaces and the discrete current carrying portions of switch must be maintained in electrically separate relationships so that there is a sea level dielectric to withstand a voltage of 1000V RMS. When it is also considered that 332 separate switches must in aggregate weigh less than one pound (the weight specification maximum is 0.003 lbs.), the difficulty of meeting the diverse specification requirements come into focus. High performance operating characteristics also obtain in that it is required that the switch must switch current with a maximum actuating force of 4 ounces, a releasing force of 7/10 of an ounce and that a movement differential be provided of 4/1000 of an inch or less, a pretravel of at least 15/1000 of an inch and there be a minimum of 3/1000 of an inch of overtravel (and more would be desirable).
Packing of all these electrical, mechanical and environmental requirements into such a tiny physical package with such high precision operating characteristics has been a formidable task for the industry. Prior hereto, despite need, no close approach to the specification 8805/94 has been provided. It is believed that one of the main problems has been to provide a switch of this tiny size which can meet the switch electrical current characteristic of 1 Amp. Another obstacle in this size switch is meeting the momentary high amperage short circuit specification characteristics without significant degrading of the contacts. Some switch configuration attempts, when prior art types of downsized relatively standard blade constructions were attempted (both flexible blade and coil spring operated), turned into what might be called a non calibrated fusible link when exposed to the high amperage short circuit requirements. In essence the whole movable contact blade mechanism melted.
A further sizable problem is to pack all of the very high precision movable parts in such a tiny space, while meeting the precision operating characteristics and simultaneously providing sufficient heft to meet the mechanical objectives of the specification.
A search was conducted on the subject matter herein and the following references were developed:
______________________________________ Date Patent No. Inventor/Assignee ______________________________________ 10/16/11 58076 (Swiss) Brevet/Ph. Morand & Fils 4/18/45 464,406 (Canada) Gratzmuller 3/9/60 829,965 (G. Br.) Leney 1/15/63 3,073,923 Anderson-Cherry/Cherry Elec- trical Products Corp. 7/14/64 3,141,075 Brevick-Cherry/Cherry Elec- trical Products Corp. 8/9/66 3,165,823 Bury/Illinois Tool Works 8/22/67 3,337,702 Brevick-Cherry.Dernehl/Cherry Electrical Products 5/7/68 3,382,332 Cherry-Grady/Cherry Electrical 9/3/68 3,400,234 Long/Cherry Electrical 3/11/69 3,432,632 Schenke/Firma J & J Marquardt 6/10/69 3,449,538 Long/Cherry Electrical 12/23/69 3,485,975 Long/Cherry Electrical 10/12/71 3,612,793 Roeser/Otto Engineering 11/30/76 3,995,129 Michalski/Rudolf Schadow GmbH 3/8/77 4,011,419 Anderson/Cherry Electrical 6/9/81 4,272,660 Mayer-Pescetto/Stewart Warner 8/3/82 4,342,885 Kashima-Takase-Mishina/ Matsushita Electric Works 6/16/87 4,673,778 Lewandowski-Doros-Redfield/ The Cherry Corporation ______________________________________
All of these references are of general interest and the three superficially closest references developed are the Cherry switch shown in 4,673,778, the Roeser/Otto Engineering switch (FIG. 17) shown in 3,612,793 and the Michalski switch 3,995,129. The 4,673,778 and the 3,995,129 do not show or teach high performance switches, nor can they be modified in any reasonable manner so as to meet high performance standards aforenoted. The Otto Engineering switch is a high performance switch but cannot be downsized to the substantially smaller switch herein and cannot be modified so as to meet the criteria above discussed without a complete change in structure and relationships.